Soft lighting through translucent material in cycles
Here's a scene before the curtain. The only light is an emission plane outside and over the window.


(clik on the images to enlarge)

If we add a translucent curtain we get a lot less light illuminating the room.



Adding transparency will allow more light into the room, but will make the curtain "see-trough".



Controlling the transparency so that is not visible to the camera will make the light come into the room while keeping the translucency of the curtain's material:



By changing the ray depth values you can control how much light comes into the room: 







Alternatively

You can just turn the whole curtain into an emission object unseen by the camera: